A mechanical coolant pump is a coolant pump which is driven by a combustion engine, for example, by using a driving belt which drives a driving wheel of the pump. For efficiency reasons, only a minimum or even no coolant flow is needed as long as the temperature of the combustion engine is low or has not reached its operating range. Switchable mechanical coolant pumps are therefore used which are provided with a clutch to couple the driving wheel with the pump wheel pumping the coolant. The clutch is disengaged as long as the combustion engine is cold so that the circulation of the coolant is minimized or stopped, with the result that the warming of the combustion engine is sped up. The clutch is switched into the engaged position when coolant circulation is required.
A known type of clutch is the mechanical friction clutch which is actuated by the interaction of a pretensioning spring, a permanent magnet, and an electromagnet, as described in EP 2 299 085 A1. The permanent magnet causes a permanent magnetic attraction force which forces the clutch into the engaged position. When the electromagnet is energized, the magnetic force of the permanent magnet is reduced so that the clutch is forced by the spring into the disengaged position. The permanent magnet is a separate small ring with a high magnetic performance, for example, a permanent magnet made of sintered neodymium. These kinds of magnets, i.e., magnets of a rare-earth material, are expensive and difficult to machine. Ferromagnetic bodies are also necessary to conduct the magnetic field between the permanent magnet and the clutch discs.